1. Field of the Invention
This invention relates to a liquid crystal display device used for a display unit such as of electronic equipment.
2. Description of the Related Art
In recent years, a liquid crystal display device of the active matrix type having a thin-film transistor (TFT) for each pixel has been widely used as a display device for a variety of applications.
FIG. 8 illustrates the constitution of a pixel in a conventional active matrix liquid crystal display device. A metal layer formed on the whole surface of the substrate is patterned to form gate bus lines 106 and storage capacitor bus lines 108. Next, a gate-insulating film (not shown), an operation semiconductor layer and a channel protection layer (both of which are not shown) are formed on the whole surface of the substrate on the gate bus lines 106 and on the storage capacitor bus lines 108, and an operation semiconductor layer and a channel protection film 110 are patterned on the gate bus lines 106. Here, the gate-insulating film is formed on the whole surface of the substrate. Next, an n+ semiconductor layer and a metal layer are formed on the whole surface of the substrate on the gate-insulating film, on the operation semiconductor layer and on the channel protection film 110, and are patterned to form source electrodes S, storage capacitor electrodes 112 and data bus lines 114.
Next, a protection film (not shown) is formed on the whole surface of the substrate on the source electrodes S, on the storage capacitor electrodes 112 and on the data bus lines 114. Next, the protection film on the storage capacitor electrodes 112 and on the source electrodes S is removed by etching to form contact holes 116 and 122. Next, pixel electrodes 118 made of a transparent material are formed. The pixel electrodes 118 are electrically connected to the storage capacitor electrodes 112 and to the source electrodes S, respectively, through the contact holes 116 and 122.
FIG. 9 is an equivalent circuit of a pixel shown in FIG. 8. Referring to FIG. 9, a parasitic capacitance Cgs is present between the gate bus line 106 and the source electrode S of a TFT 120, parasitic capacitances Cds1 and Cds2 are present among the two parallel data bus lines 114 and the pixel electrode 118, and a parasitic capacitance Cs is present between the storage capacitor bus line 108 and the pixel electrode 118. Further, the pixel electrode 118 or the liquid crystal cell is expressed by a parallel circuit of a liquid crystal capacitance CLC and a liquid crystal resistance RLC.
FIG. 10 illustrates voltage waveforms of a gate pulse voltage Vg applied to the gate bus line 106 in the pixel shown in FIGS. 8 and 9, of a data signal voltage Vd applied to the data bus line 114 and of a pixel voltage Vp applied to the pixel electrode 118. FIG. 10 shows voltage waveforms of when the data signal voltage Vd is applied to the neighboring data bus lines 114 in reverse polarities, the data signal voltage Vd being a voltage waveform applied to the data bus line 114 on the left side in FIGS. 8 and 9. Further, the gate pulse voltage Vg shown in FIG. 10 is a voltage waveform applied to the gate bus line 106 on the upper side in FIGS. 8 and 9.
Referring to FIG. 10, a high-level period after the gate pulse voltage Vg applied to the gate bus line 106 has changed into Vgon from Vgoff, is a selection period Ton in which the TFT 120 is turned on. A low-level period after the gate pulse voltage Vg has changed into Vgoff from Vgon is a non-selection period Toff in which the TFT 120 is turned off. In the selection period Ton, the TFT 120 is turned on, and the data signal voltage Vd of positive polarity applied to the data bus line 114 is further applied to the pixel electrode 118 through the source electrode S of TFT 120. Thereafter, when the gate pulse voltage Vg falls from Vgon to Vgoff, the potential of the pixel electrode 118 decreases by ΔV (Cgs) as shown in FIG. 10 due to the parasitic capacitance Cgs.
[Patent document 1 ]JP-A-10-301140
When a distance (left side distance d1) between the pixel electrode 118 and the data bus line 114 on the left side in FIGS. 8 and 9 is equal to a distance (right side distance d2) between the pixel electrode 118 and the data bus line 114 on the right side in the drawings, the pixel voltage Vp of the pixel electrode 118 assumes a nearly constant voltage waveform in the non-selection period Toff as shown in FIG. 10. However, the data bus line 114 and the pixel electrode 118 are formed by different layers. In practice, therefore, an overlapping error occurs due to dispersion in the production process, and a difference occurs between the left side distance d1 and the right side distance d2 in the display surface.
FIG. 11 shows a voltage waveform such as of the pixel voltage Vp or the like of when the left side distance d1 is shorter than the right side distance d2. When the left side distance d1 is shorter than the right side distance d2, the capacitance value of the parasitic capacitance Cds1 that takes place between the pixel electrode 118 and the data bus line 114 on the left side in FIG. 9 becomes greater than the capacitance value of the parasitic capacitance Cds2 that takes place between the pixel electrode 118 and the data bus line 114 on the right side. Therefore, in the non-selection period Toff in which the TFT 120 is turned off, as shown in FIG. 11, when the data signal voltage Vd of the data bus line 114 changes from a voltage Vdh of positive polarity into a voltage Vdl of negative polarity or vice versa, the pixel voltage Vp varies by ΔV (Cds) following the change in the data signal voltage Vd of the data bus line 114 on the left side due to the parasitic capacitance Cds1. For example, the pixel voltage Vp decreases by ΔV (Cds) at a moment when the data signal voltage Vd of the data bus line 114 of the left side falls to the voltage Vdl of negative polarity from the voltage Vdh of positive polarity.
FIG. 12 shows a voltage waveform such as of the pixel voltage Vp of when the left side distance d1 is longer than the right side distance d2. When the left side distance d1 is longer than the right side distance d2, the capacitance value of the parasitic capacitance Cds2 becomes greater than the capacitance value of the parasitic capacitance Cds1. Therefore, in the non-selection period Toff in which the TFT 120 is turned off, as shown in FIG. 12, when the data signal voltage Vd of the data bus line 114 changes from a voltage Vdh of positive polarity into a voltage Vdl of negative polarity or vice versa, the pixel voltage Vp varies by ΔV (Cds) following the change in the data signal voltage Vd of the data bus line 114 on the right side due to the parasitic capacitance Cds2. For example, the data signal voltage Vd of the data bus line 114 on the right side rises from the voltage Vdl of negative polarity to the voltage Vdh of positive polarity at a moment when the data signal voltage Vd of the data bus line 114 of the left side falls down to the voltage Vdl of negative polarity from the voltage Vdh of positive polarity. Therefore, the pixel voltage Vp increases by ΔV (Cds).
Here, if the amount of change in the data signal voltage Vd is denoted by ΔVd, then, ΔV(Cds) is expressed as follows:ΔV(Cds)=|(ΔVd·Cds1/(Cs+CLC+Cds1+Cds2)−ΔVd·Cds2/(Cs+CLC+Cds1+Cds2)|  (1)
When there occurs a difference between the left side distance d1 and the right side distance d2, there occurs a difference between the capacitance value of the parasitic capacitance Cds1 and the capacitance value of the parasitic capacitance Cds2, too, causing the pixel voltage Vp to vary. That is, when the difference between the left side distance d1 and the right side distance d2 in a predetermined region A on the display surface is not the same as the difference between the left side distance d1 and the right side distance d2 in a predetermined region B, the pixel voltage Vp applied to the pixels deviates from a desired voltage in different amounts depending upon the regions A and B. Therefore, shading in the brightness is seen on the display screen causing a conspicuous decrease in the quality of display of the liquid crystal display device.